The present invention relates to an operating state circuit for an inverter and a method for setting operating states of an inverter, in particular for an inverter for supplying power to a synchronous machine.
Electric drives of hybrid or electric vehicles may include permanently excited electric machines, in particular synchronous machines, which are supplied with AC voltage by means of a pulse-controlled inverter. In the event of fault states on the low-voltage side, for example, in the event of a failure of the electronic control, the energy supply, various sensors, the data communication, or a fuse, it may be necessary to set a safe state in the inverter i.e., a switching state of the inverter in which safety remains ensured for persons accessing the vehicle such as first responders in the event of an accident, and for the integrity of the electrical system.
Generally, a control device of the inverter is supplied by the low-voltage system of the vehicle. In the event of a failure of the low-voltage supply, a control device of the inverter may switch the inverter independently to a safe state in order to prevent possible damage to electrical components, for example, as a result of an uncontrolled energy input into the DC voltage intermediate circuit from which the inverter is fed. Various circuit states may be achieved in conventional methods.
For example, all switches connected to the low potential, so-called low-side switches, may be closed, and all switches connected to the high potential, so-called high-side switches, may be opened. This operating mode is also referred to as a short-circuit state to low potential. Alternatively, all high-side switches may be closed, and all low-side switches may be opened, so that a short-circuit state to high potential is created. In another shutdown method, all switches of the pulse-controlled inverter are opened. This is also referred to as freewheeling mode.
For example, a combination of shutdown methods is known from publication DE 10 2006 003 254 A1. For example, since the phase current may still rise for a short time after switching to the short-circuit state, it is provided there to use both known shutdown operating modes sequentially and to switch the electric machine initially to freewheeling mode and subsequently to the short-circuit state.
Publication DE 10 2009 047 616 A1 describes an inverter circuit for an electric machine which is able to be transferred in the event of a fault from an active short circuit operating state to a freewheeling operating state.
Publication WO 2012/000710 A2 describes a method for operating an at least three-phase electric machine used as a drive assembly in a motor vehicle, which is actuated via an inverter, in particular a pulse-controlled inverter, the inverter including switching elements in the form of half bridges, and in each case a half bridge being electrically connected to a phase of the electric machine. If a first switching element of a first half bridge connected to a first phase is permanently closed on account of failure, then, according to the present invention, a second switching element of the first half bridge is permanently opened and, in a first angular range of an electrical rotation, which is uninfluenced by the first phase, all switching elements of the additional half bridges are actuated in the usual manner.
In a short-circuit state, a braking torque forms which is generated by the generator operation of the synchronous machine. For example, if the car is towed, the braking torque is a hindrance. In particular at low rotational speeds, the braking torque of the electric machine caused by the short-circuit state is relatively high. Publication JP 60005791 A1, for example, provides for using a torque limiter which is controllable as a function of the intermediate circuit voltage to limit the braking torque, in order to prevent overloading of the intermediate-circuit capacitor.
There is therefore a need for solutions for controlling an inverter in an electric drive system including an electric machine, in which operating states of the inverter may be set in a safe and efficient manner even in the event of a failure of the low-voltage supply of the controller.